Probably around the same time so they can sell two copies of the game as people upgrade and realize they are without the most bought and played game of the next decade. Rockstar has a winning model.
The systems seem to be lasting about 7 years now, if that trend continues...you would be expecting GTA 6 to take a minimum of over 14 years from V's release date for the PS6 to beat it. Having a hard time seeing that. People still can't sniff a next-gen system, I wouldn't be surprised if the shortages / pandemic expand the lifespan of the systems a few years